1. Field of the Invention
The invention relates to an apparatus for on-line detection and measurement of cold emulsion adhesives applied to a substrate based on determining near infrared light energy absorption at two specific wavelengths, one being within an absorption band of water and the other being outside the absorption band of water.
2. Discussion of Related Art
On-line measurement of key glue-line characteristics of corrugated board is known from U.S. Pat. No. 5,663,565 (the '565 patent), entitled SYSTEM AND METHOD FOR THE ON-LINE MEASUREMENT OF KEY GLUE-LINE CHARACTERISTICS ON CORRUGATED BOARD and issued Sep. 2, 1997 and whose contents are incorporated herein by reference. According to its disclosure, the measurement signal is typically generated by either an infrared absorption sensor, which is used to measure the mass of moisture and/or starch in the glue-line, or an infrared pyrometer, which is used to measure the temperature of the glue-line. The chosen sensor is installed so as to measure the exposed glue-line prior to bonding of the medium with a liner. In addition, the disclosure reveals sensing isolated glue-based variations found in the sensor output signal that oscillate at a known fluting frequency of the corrugated board.
Variation in the sensor output signal that are not oscillating at the fluting frequency are canceled out, while those that are oscillating at the fluting frequency are isolated and amplified. An incremental amount of infrared radiation that is absorbed by starch and/or water in the glue-lines is isolated from the predominant, more random background absorption due to cellulose and water in the paper substrate. The amplitude of the extracted signal component, which reflects only starch and/or water in the glue, is then converted using empirically derived calibration constants into a final starch mass value. The output signal of an infrared absorption sensor, therefore, provides an improved on-line starch measurement for corrugators.
While the '565 patent mentions that to date no on-line glue measurement techniques have been successfully commercialized, the present inventors believe that have found a solution. The conventional problem with glue application on substrates lies with detection and correction of irregularities, which the '565 patent identifies and elaborates upon in some detail. However, the present inventors have devised a technique to discriminate the adhesive from its substrate and to quantify the moisture content of the substrate and adhesive to promote a more uniform glue application.